The present invention relates to an electrophotographic type image forming apparatus in which a charging means, an image exposure device and a developing means are arranged around an image forming body for image formation, in an image forming apparatus such as copiers, printers, facsimiles, or similar apparatus. Specifically, the present invention relates to a holding structure for an image forming body, rotated in the subsidiary scanning direction in an electrophotographic type color image forming apparatus, in which a plurality of chargers, image exposure devices and developing devices are arranged around an image forming body, and whereby toner images are superimposed on the image forming body during a single rotation of the image forming body for color image formation.
As a multi-color image forming apparatus, the following apparatuses (A), (B) and (C) are generally known:
(A) A color image forming apparatus in which the same number of photoreceptors, chargers, developing devices as the number of necessary colors are provided, whereby a monochromatic toner image formed on each photoreceptor is superimposed on an intermediate transfer body, or similar devices, to form a color image. PA1 (B) A color image forming apparatus in which a single photoreceptor is rotated plural times, and charging, image exposure, and developing are repeated for each color to form a color image. PA1 (C) A color image forming apparatus in which charging, image exposure, and developing are successively conducted for each color during one rotation of a single photoreceptor to form a color image.
However, the apparatus (A) requires a plurality of photoreceptors and an intermediate transfer body, resulting in a disadvantageous increase in overall size of the apparatus. The apparatus (B) has only one charging means, image exposure means, and photoreceptor, and therefore, the overall size of the apparatus is decreased. However, apparatus (B) has a limitation in that the size of the image is limited to less than the surface area of the photoreceptor. The apparatus (C), moreover can form an image at high speed, but it requires that a plurality of sets of a charger, an image exposure means, and a developing device are provided around the periphery of the photoreceptor. Further, in the apparatus (C), there is the possibility that the optical system which conducts image exposure is stained by toner leaked from the adjoining developing device, which reduces the image quality. In order to avoid that, it is required to enlarge the interval between the image exposure means and the developing device, whereby the diameter of the photoreceptor is inevitably increased, resulting in an increase of the overall apparatus, which is a problem.
In order to avoid the disadvantages of the apparatus (C), an apparatus (D) has been proposed, (Japanese Patent Publication Open to Public Inspection No. 307307/1993), in which the base body of the image forming body is formed of a transparent material; a plurality of image exposure means are housed inside the image forming body; and an image is exposed on a photoreceptor layer formed on the outer periphery of the image forming body through the transparent base body.
The image forming apparatus (C) and (D) can form a color image during a single rotation of the image forming body. As a result, the image recording time period can be shortened, so that high speed recording can be carried out. This apparatus is also effective for an increase of image quality.
In the above image forming apparatus (D), an apparatus of the type (hereinafter, which is called the optical system including type) has been proposed in which a linear image exposure means for each color is arranged inside the image forming body composed of a transparent base body and an optical semiconductor. The image forming apparatus having the optical system included-type exposure means has an advantage in which the overall size of the image forming body can be reduced, so that the apparatus is structured more compactly.
In the image forming apparatus having the optical system included-type exposure means, a light collection position by each linear light emitting means accurately coincides with the image forming surface on the peripheral surface of the image forming body, and arrangement positions of linear light emitting means are required to be accurately parallel to each other with a predetermined interval. Accordingly, conventionally, each linear exposure optical system is temporarily attached to an optical system supporting body, color image processing is carried out using the image forming devices housed in the image forming body, image quality of the formed image is checked, and the attached position of each linear light emitting means is then corrected. The adjusting operation for this position correction requires a technician's skill and a long period of time, which is a problem for assembling the apparatus. Alternatively, each linear exposure optical system is attached to the optical system supporting body, and position adjustment and focus adjustment are carried out using optical system assembling jigs. Then the linear exposure optical system is fixed onto the optical system supporting body, and after the image forming body and a position regulation member (bearing member) are arranged, these assemblies are mounted at a predetermined position in the image forming apparatus. However, in the above conventional adjustment, even when the adjustment is correctly carried out by the optical system assembling jigs, when the linear exposure optical system is mounted into an actual image forming apparatus, a problem occurs in that the accuracy of the focus of the optical system, and the positional accuracy in the primary scanning direction or in the subsidiary scanning direction are lowered due to the accuracy of the position regulating member or assembling errors of the optical supporting body itself. As a result, re-adjustment is required after the optical systems have been mounted into the actual apparatus.
In conventional systems, the position of the shaft of the supporting body is a reference position, and therefore, it is necessary that this position of the shaft is very accurately determined for assembling. For example, as shown in FIG. 14(A), if the shape of the supporting body SPT itself is not accurate, when an optical system LED is mounted onto the supporting body SPT, the LED is moved in the arrowed direction and the mounting position is adjusted so that the optical system LED is focused on a sensor S, and is mounted on the supporting body SPT through a spacer SP having an appropriate thickness, using a reference pin PIN corresponding to a shaft hole H of the supporting body and a mounting jig J having a mounting arm ARM. Accordingly, distance "a" between an axis A of the supporting body SPT and the sensor S is constant.
After the LED has been thus mounted onto the supporting body SPT, when a bearing B and a photoreceptor PR are mounted onto the supporting body SPT, as shown in FIG. 14(B), an actual rotation center C is determined by the bearing, and there is a deviation between the rotation center C and the axis A of the supporting body SPT. As a result, a distance "d" in FIG. 14(B) fluctuates, and the LED can not be accurately focused on the surface of the photoreceptor PR corresponding to the mounting position of the sensor S.
That is, when the shape of the supporting body itself is not accurate as shown in FIG. 14(A), the optical system (LED or the like) can not be accurately mounted with respect to the rotation center. In other words, there is a problem in that the distance between the photoreceptor and the optical system fluctuates (conventionally, this is due to problems with respect to the tolerance of parts or the tolerance of assembly), and therefore, inevitably, material is required to be processed more accurately, or it is inevitable to disregard several fluctuations.
This results from the fact that the actual rotation center is not used as the reference. Because the optical system and its supporting member are not necessary to be rotated, it is necessary that the rotation member, which actually contributes to the rotation, is used as the reference, and this point is improved in the present invention.
An image forming body which is located at an image forming position of the plurality of exposure means, and on which superimposed images are formed by a plurality of exposure means, is required to be accurately held so that the image does not deviate in the primary scanning direction, and to be rotatably accurately supported so that the focus is not shifted.
Conventionally, an ordinal radial bearing is used for a bearing member which holds the rotating image forming body. However, in the radial ball bearing, movement errors tend to occur in the thrust direction, so that mechanical play of several 10 .mu.m occurs in the primary scanning direction of the image forming body. When a color image is formed by using the image forming body supported by such bearing member, doubling occurs in the superimposed images.